1. Field of the Invention
The present invention relates to a ceramic substrate for electronic circuit using, as a conductive metal, copper or an alloy containing copper as its main component and a method for producing the same. More particularly, it relates to a ceramic substrate for electronic circuit in which a special composition is used for a ceramic layer in a ceramic substrate for multilayer electronic circuit or for an insulated layer in a ceramic substrate for printed electronic circuit.
2. Description of the Prior Art
In recent years, there has been a great demand for a ceramic substrate for electronic circuit since it is required that a circuit element or a semiconductor device be highly integrated and a circuit substrate be made highly reliable.
As a ceramic substrate for electronic circuit, there are a substrate for multilayer electronic circuit obtained by patterning a conductive layer on unfired ceramic green sheets, laminating the resulting green sheets, followed by firing them together; a substrate for printed electronic circuit obtained by forming a conductive layer, an insulated layer, and another conductive layer on a sintered ceramic substrate in this order so that the insulated layer is sandwiched between both conductive layers; and the like. The substrate for multilayer electronic circuit is used as a substrate which is required to be highly integrated such as a circuit substrate on which an LSI chip or the like is mounted. The substrate for printed electronic circuit is used as a substrate of a hybrid IC or the like in which functional elements such as a resistor and a capacitor are further formed on the substrate.
In the past, as a ceramic material used for ceramic substrates for electronic circuit, an alumina type material, a glass ceramic type material, or the like has been used. In particular, as a method for a ceramic substrate similar to the present invention, Japanese Patent Publication Nos. 51-11795, 51-11793, 51-26640, etc. disclose materials obtained by adding a copper oxide to alumina is used.
As a material for a conductive layer, tungsten, molybdenum, or the like is used in the case of a substrate for multilayer electronic circuit; and a gold or silver type material is used in the case of a ceramic substrate for printed electronic circuit. However, tungsten, molybdenum, or the like has a large electric resistance, causing problems when wiring is made thin. Moreover, there are the problems in that gold is expensive and silver is likely to migrate.
In order to solve the above-mentioned problems, a ceramic substrate for electronic circuit in which inexpensive copper having a low resistance is used for a conductive layer has been proposed. When a conductive layer and a ceramic layer in the case of a substrate for multilayer electronic circuit are simultaneously fired, or when a conductive layer and an insulated layer in the case of a substrate for printed electronic circuit are simultaneously fired, using e base metal such as copper as the conductive material, a binder in the ceramic layer or the insulated layer and the conductive layer is burnt out by adding water vapor in an inert gas atmosphere in which the conductive layer is not oxidized, and then the ceramic layer or the insulated layer and the conductive layer are fired in an inert gas atmosphere. However, according to this method, the binder is not sufficiently burnt out, causing problems such as remaining carbon. For the purpose of solving this problem, as described in Japanese Laid-Open Patent Publication No. 61-26293, a method comprising the steps of: forming the conductive layer with a copper oxide; burning out the binder in the ceramic layer or the insulated layer and the conductive layer in air; metallizing the conductive layer alone at a low temperature by reduction; and firing the ceramic layer or the insulated layer and the conductive layer in an inert gas atmosphere.
Moreover, as described in U.S. Pat. No. 4,503,090, a method comprising the steps of: diffusing a copper component oxidized from the conductive layer into the ceramic layer on the lower part of the conductive layer; and forming a reaction layer of copper oxide and a ceramic component on the lower part of the conductive layer has been proposed.
However, in a conventional ceramic substrate for electronic circuit having the conductive layer comprising copper or an alloy containing copper as its main component, the copper oxide is partially diffused from the conductive layer to the ceramic layer in the case of the substrate for multilayer electronic circuit or in the insulated layer in the case of the substrate for printed electronic circuit in the course of the production. Thus, defects are caused due to the difference in the firing conditions between the portion in which the copper oxide is diffused and the other portion.